Nonlinear spectral-like schemes for hybrid schemes

In spectral-like resolution-WENO hybrid schemes, if the switch function takes more grid points as discontinuity points, the WENO scheme is often turned on, and the numerical solutions may be too dissipative. Conversely, if the switch function takes less grid points as discontinuity points, the hybrid schemes usually are found to produce oscillatory solutions or just to be unstable. Even if the switch function takes less grid points as discontinuity points, the final hybrid scheme is inclined to be more stable, provided the spectral-like resolution scheme in the hybrid scheme has moderate shock-capturing capability. Following this idea, we propose nonlinear spectral-like schemes named weighted group velocity control (WGVC) schemes. These schemes show not only high-resolution for short waves but also moderate shock capturing capability. Then a new class of hybrid schemes is designed in which the WGVC scheme is used in smooth regions and the WENO scheme is used to capture discontinuities. These hybrid schemes show good resolution for small-scales structures and fine shock-capturing capabilities while the switch function takes less grid points as discontinuity points. The seven-order WGVC-WENO scheme has also been applied successfully to the direct numerical simulation of oblique shock wave-turbulent boundary layer interaction

Plasma-assisted ignition for a kerosene fueled scramjet at Mach 1.8

By using a plasma jet (PJ) torch with 1.5 kW input power as an igniter, successful ignition for liquid-kerosene fueled combustion experiment was conducted in a direct-connected supersonic test facility. The incoming flow has total temperature of 950 K and local Mach number of 1.8, corresponding to Mach 4 flight condition. In this study, several optical techniques, including high speed photography, high speed schlieren photography, and planar laser scattering (PLS) technique, were combined to study the ignition process, flame propagation, and mixing features of liquid kerosene fuel with air around the cavity. The effect of fuel injection position, injection pressure, and feedstock gas on ignition performance has been analyzed. The results indicate that local mixing is a critical factor for ignition. It is also shown that the PJ torch with N-2 + H-2 feedstock is superior to the PJ torch with N-2 feedstock for the ignition of liquid-kerosene fuel. These results are valuable for the future optimization of kerosene-fueled scramjet engine when using a PJ torch as an igniter

Numerical Study on Wall Temperature Effects on Shock Wave/Turbulent Boundary-Layer Interaction

The effect of wall temperature on the size of the separation bubble in the shock wave/turbulent boundary-layer interaction of a 24 deg compression ramp with Mach 2.9 is numerically investigated. The ratios of wall temperature to recovery temperature T-w/T-r are 0.6, 1.14, 1.4, and 2.0, respectively. To validate the simulation, the statistical results with T-w/T-r = 1.14 are tested and the results show a good agreement with theoretical and experimental results. It is shown that wall temperature has a remarkable effect on the size of the separation bubble and the size increases significantly with the increase of wall temperature. Through theoretical analysis, combined with numerical results, we get a semitheoretical formula L/delta alpha (T-w/T-r )(0.85), in which L and delta are the length of the separation bubble and the thickness of upstream boundary layer, respectively. The turbulent kinetic energy budgets are also analyzed based on the numerical data, and results show that turbulence kinetic energy is chiefly produced both in the buffer layer and near the shock wave, and turbulent dissipation is mainly in the center of the separation bubble as well as in the near-wall region. It is also shown that the intrinsic compressibility effect is not significant in all these cases

Skeletal mechanism modelling of n-heptane/oxygen laminar coflow flame structure at pressures

A skeletal reaction mechanism of n-heptane combustion is developed and validated. The axisymmetric laminar co-flow diffusion flames of n-heptane/oxygen are simulated using the skeletal mechanism at elevated pressure, and the chemical reaction paths in three reaction zones of flame are presented. The n-heptane/oxygen flame height decreases from 8.8 to 5.6 mm with the increase of pressure from 0.1 to 2 MPa, and the Roper's formulation is not expected for the prediction of n-heptane/oxygen flame height. The maximum volume fraction of soot increases with pressure as f(v,max) proportional to P-2. The ratio of flame height to soot oxidation length of the n-heptane/oxygen flames is close to unity (1.037-1.121). (C) 2015 Elsevier Ltd. All rights reserved

Skeletal mechanism modelling of n-heptane/oxygen laminar coflow flame structure at pressures

A skeletal reaction mechanism of n-heptane combustion is developed and validated. The axisymmetric laminar co-flow diffusion flames of n-heptane/oxygen are simulated using the skeletal mechanism at elevated pressure, and the chemical reaction paths in three reaction zones of flame are presented. The n-heptane/oxygen flame height decreases from 8.8 to 5.6 mm with the increase of pressure from 0.1 to 2 MPa, and the Roper's formulation is not expected for the prediction of n-heptane/oxygen flame height. The maximum volume fraction of soot increases with pressure as f(v,max) proportional to P-2. The ratio of flame height to soot oxidation length of the n-heptane/oxygen flames is close to unity (1.037-1.121). (C) 2015 Elsevier Ltd. All rights reserved

Release of alkali metals during co-firing biomass and coal

The release of alkali metals is investigated by means of ash element measurement and chemical equilibrium calculation during co-firing biomass (cornstalk derived fuel) and coal. The experiments are operated under different mass blending fraction and temperature conditions. With increasing cornstalk fraction, the normalized ash content decreases according to a quadratic curve because the formation of alkali aluminosilicate may result in more elements being retained in ash. Similarly, the release ratios of K, Na, and S do not change linearly with the variation of cornstalk fraction either, which are suppressed significantly by the interaction of biomass and coal ashes. For the effect of CaO additive, the release ratios of K and Na increase with its enhancement. The equilibrium analysis is used to predict and evaluate the release of alkali metals. The release of alkali metals enhances with the increase of temperature and cornstalk fraction. For silicon-lean blending fuels, adding Ca will decrease HCl(g) and release more KCl(g) and KOH(g) in both fuel-rich and air-rich conditions. More KCl(g) and NaCl(g) will be formed with increasing Ca/(S + 0.5Cl) ratio due to the formation of CaSiO3(s) and the reduction of alkali aluminosilicate. (C) 2016 Elsevier Ltd. All rights reserved

Catalytic reduction of nitric oxide by carbon monoxide over coal gangue hollow ball

The catalytic effect of coal gangue hollow ball with open pores on NO reduction by CO in fluidized bed was investigated, and the kinetics for the catalytic reduction of NO using CO was evaluated. In the absence of water vapor at 773-923 K, the reaction rate constant was k = 5210 exp(-113600/(RT)). NO reduction ratio rapidly increased with reaction residence time at initial stage (<1 s) and T > 873 K; high NO reduction ratio was achieved by increasing reaction temperature when the initial concentration ratio of CO to NO was less than 4. In the presence of water, the catalytic effect of coal gangue hollow ball on NO reduction was weakened, and the reaction rate constant was k = 0.31 exp(-60750/(RT)). NO reduction ratio is less than 02 when reaction residence time was 2.5 s at T <= 923 K when the initial concentration ratio of CO to NO was 4, and the initial concentration ratio of CO to NO had negligible effect on NO reduction ratio at T <= 923 K. (C) 2014 Elsevier B.V. All rights reserved

Mixing and combustion modeling of hydrogen peroxide/kerosene shear-coaxial jet flame in lab-scale rocket engine

H2O2/kerosene bipropellants are effective alternatives for toxic propellants currently in use. The complex H2O2/kerosene turbulent flow and combustion chemical reaction are modeled, the atomization, evaporation and mixing are investigated, the characteristics of the complex chemical reaction path of kerosene at different stages in combustion chamber are analyzed, and the profiles of static pressure, temperature and Mach number of exhaust plume are provided. During the kerosene injection, the spray angle is 13 degrees, the atomization and evaporation occur and complete simultaneously, the lasting time of atomization is about 1 ms, and the maximal vaporization rate reaches 1.5 x 10(-6) kg/s. The combustion chamber can be divided into the following three zones: rapid high-temperature pyrolysis zone, oxidization zone and equilibrium flow zone. The initial excitation of kerosene reactions is crucial link, and the hydrogen abstraction reactions are the dominant reactions in the high pyrolysis. These peak values of static pressure, temperature, Mach number along plume axis almost evenly distribute, and the interval distances are about 32 mm along the centerline of axis. (C) 2016 Elsevier Masson SAS. All rights reserved.</p

Three-dimensional flame measurements with large field angle

A system for three-dimensional computed tomography of chemiluminescence was developed to measure flames over a large field angle. Nine gradient-index rods, with a 9 x 1 endoscope and only one camera are used. Its large field of view, simplicity, and low cost make it attractive for inner flow field diagnostics. To study the bokeh effect caused by the imaging system on reconstruction solutions, fluorescent beads were used to determine the blurring function. Experiments using a steady diffusion flame were conducted to validate the system. Three models, namely the clear-imaging, out-of-focus imaging, and deconvolution models, were utilized. Taking the bokeh effect into account, the results suggest that based on run-times the deconvolution model provides the best reconstruction accuracy without increasing computational time. (C) 2017 Optical Society of America;A system for three-dimensional computed tomography of chemiluminescence was developed to measure flames over a large field angle. Nine gradient-index rods, with a 9 x 1 endoscope and only one camera are used. Its large field of view, simplicity, and low cost make it attractive for inner flow field diagnostics. To study the bokeh effect caused by the imaging system on reconstruction solutions, fluorescent beads were used to determine the blurring function. Experiments using a steady diffusion flame were conducted to validate the system. Three models, namely the clear-imaging, out-of-focus imaging, and deconvolution models, were utilized. Taking the bokeh effect into account, the results suggest that based on run-times the deconvolution model provides the best reconstruction accuracy without increasing computational time. (C) 2017 Optical Society of America</p

Measurements of spectral parameters for nitrous oxide near 4.56 mu m using a quantum cascade laser

Line strengths and nitrogen (N-2)-broadening coefficients for six nitrous oxide transitions were measured using a continuous-wave quantum cascade laser (cw-QCL) operating near 4.56 mu m. The temperature dependence of the exponent n for the N-2-broadening coefficients was determined over the range 298-800 K using a sapphire-sealed optical cell. Spectral parameters were determined by fitting absorption spectra with multi-peak Voigt profiles. The line strengths for the six transitions are 0-3% larger than those in the HITRAN 2012 database, while the N-2-broadening coefficients at the reference temperature are 2-5% smaller than the HITRAN 2012 values. (C) 2016 Elsevier Ltd. All rights reserved